1. Field of the Invention
This invention relates generally to data transmission networks and more particularly to determining the data rates of a telephone network used for high speed data services.
2. Description of Related Art
With the recent growth in the use of the internet and expansion of small and home offices, there has been a great demand for high speed data services at numerous premises. Many phone companies have responded to this demand by offering DSL services over their existing telephone lines. DSL stands for xe2x80x9cDigital Subscriber Loop.xe2x80x9d
DSL services allow information to be transmitted over the phone line in digital form. With the digital encoding used in a DSL system, the number of bits that can be transmitted through a phone line is much greater than with a traditional analog modem. An analog modem provides a carrier signal that is modulated with the bits that represent the digital information. Modulation can be in amplitude, frequency or phase.
Because the modulating information is digital, the carrier can be modulated into one of several states. To transmit one bit at a time, the carrier only needs to be modulated into one of two states, with one state signaling that the bit has a 0 value and the other state signaling that the bit has a 1 value.
If it is desired to transmit more than one bit at a time, the carrier can be modulated into more than two states. For example, if the carrier is modulated into one of four states, two bits can be transmitted at one time. For three bits, eight modulation states are required. Whatever modulation scheme is used, a modem at the receiving end of the line detects the modulation state of the carrier and outputs a group of bits that have a value assigned based on the modulation state.
Generally increasing the number of modulation states of the carrier increases the rate at which bits can be communicated through the channel. However, the number of bits that can be simultaneously transmitted can not be set arbitrarily large. As the number of modulation states increases, the difference between each modulation state gets smaller. The less difference there is between the modulation states, the more likely it is that noise in the communication channel will disrupt communication. Noise can make a signal in one modulation state look as it is was modulated into a different state and the receiving modem will associate the signal in that channel with the wrong modulation state. Thus, the number of bits that can be simultaneously communicated on one carrier is limited because of the noise in the communication channel.
The number of bits that can be transmitted over a communication channel is also dependent on the attenuation in that channel. If the signal is attenuated, there is less energy available for the receiving modem to distinguish between the signal and the noise. Thus, the bit rate of a channel is also limited by the amount of attenuation in the channel.
To determine what bit rate can be supported, a conventional analog modem often goes through a xe2x80x9ctraining sequence.xe2x80x9d A modem attempting to initiate a connection to another modem will often try to transmit with several different modulation schemes. The modulation scheme that provides the highest bandwidth is selected for communication between those two modems.
A DSL system operates on a principle that is similar to a traditional analog modem. However, a DSL system may use multiple carrier frequencies. As part of the training sequence, the modem determines the number of modulation states that each carrier signal can carry. Those carrier frequencies that have a high attenuation or that have a lot of noise will be assigned a small number of modulation states. Some frequencies might not be used for communication at all, if they are too noisy or have too much attenuation.
A second difference between DSL and a traditional analog modem is that many of the carrier frequencies used by the DSL modem are at higher frequencies than used in a traditional analog modem. Having higher frequency signals allows more carrier signals in one phone line and therefore more bits to be simultaneously transmitted. However, traditional telephone lines were designed for voice signals that have a frequency range below approximately 20 kHz.
While most telephone lines can carry signals at frequencies higher than 20 kHz, the attenuation of a line generally increases with frequency, which limits the bit rate that can be transmitted through the line. Not all telephone lines have the same attenuation characteristics. Some lines will be able to carry enough high frequency signals that they can support DSL service at bit rates of many hundreds of kilobits per second. Others will be able to support DSL service, but at lower bit rates. Still others will not be able to carry enough high frequency signals that they could meet the lowest level of what would be considered acceptable as a high speed data service.
To offer DSL services, it is important for the phone company to know before it connects a particular subscribed for DSL service, whether the lines running to that subscriber""s premises will support high speed data services. And, if the telephone company offers different service levels for DSL service based on the data rate selected by the customer, it is important for the phone company to know whether the lines support the minimum bit rate associated with the selected service level.
Therefore, there is a need for the telephone company to xe2x80x9cprequalifyxe2x80x9d a subscriber line for high speed data services. xe2x80x9cPrequalificationxe2x80x9d means that the telephone company determines, with a reasonable degree of certainty, that the line will support DSL service at the selected data rate and that this qualification is made before the service in installed.
One simple way that prequalification has been done is through the use of a technician in the field. A technician might actually go to the subscriber premises and attach measurement equipment to the far end of the subscriber line. Through a series of interactions with test equipment at the near end of the line, the technician could determine the data rate that could be supported.
Traditionally, the attenuation of the line at 300 kHz is used as a metric to pre-qualify subscriber lines. Though DSL service operates with multiple carrier frequencies that might range from 10 kHz to over 1 MHz, a large portion of the data transmitted at any time is generally done so on carriers with frequencies near 300 kHz. Further, the performance of the line at higher frequency is usually correlated to its performance at 300 kHz, so knowing performance at that frequency often allows an acceptable prediction of performance to be made across the full frequency spectrum.
However, sending a technician to the far end of the line to measure attenuation can be costly. Most telephone companies prefer a pre-qualification approach that does not require two-ended measurements.
An alternative approach to doing pre-qualification is to make an estimate of the length of the subscriber""s phone line because attenuation of a line is heavily influenced by the length of the line. Phone companies have developed a relationship between what is called Equivalent Working Length (EWL) and the data rate for DSL services the phone line will provide. The EWL is defined in ANSI standard T1.417-2001 Spectrum Management for Loop transmission Systems.
Techniques for making estimates of EWL without two-ended measurements have been used. One approach is to estimate the length of the line from installation information. Some simple techniques use geographic location of the subscriber in relation to the switch to which their subscriber line is connected. A simple way to estimate the subscriber""s location is through the use of postal codes. The post office assigns codes based on geography, making it possible to estimate distance between the subscriber and the switch.
Use of postal codes is not, however, very accurate. The postal code only gives an approximation of distance. If the line travels a circuitous route between the switch and the subscriber, there could be significant differences between the pre-qualification prediction of performance and actual results. Likewise, if the line has anomalies on it, such as if it is made of a smaller gauge wire than a nominal subscriber line or contains bridged taps or load coils, there could be a significant difference between the actual performance and the performance as predicted by the line length estimate.
A variation on the approach of pre-qualifying a line through postal codes is to use cable records of the telephone company. Most telephone companies have records that indicate the construction of their network. From these records, the EWL of the line might be determined. However, it has been found that many telephone company records are not up to date. Repairs or changes in the wiring sometimes does not get accurately recorded in the database. And, at some telephone companies, the cable records are partially or totally kept in paper form, making it very time consuming to locate the required information.
A further variation in using line length to pre-qualify lines for DSL service is to measure the capacitance of the line and use the measured capacitance to predict the line length. The capacitance of a line will increase as the line length increases. However, factors other than line length also influence the capacitance of the line. For example, the thickness of the wire used to make the line and the number of bridged taps on the line can influence capacitance, but usually do not have a proportionate impact on the attenuation of signals carried by the line. As a result, estimation of DSL performance of a line based on measured capacitance is not as accurate as desired.
There are other non-capacitance based techniques (such as TDR) to measure loop length for line qualification. These methods suffer from limitations similar to above mentioned capacitance based length measurement. The TDR length is also not as accurate as capacitance based length.
An improved approach to pre-qualification is with a product called Celerity(trademark), sold by Teradyne, Inc. of Deerfield, Ill., USA. That system provides a measurement unit that is attached to multiple subscriber lines through a telephone company switch. Telephone switches have traditionally been designed to allow test access to the subscriber lines. However, the test access port of telephone switches has a low frequency response meaning that only relatively low frequency signals can be coupled through the switch between the measurement unit and a particular subscriber line under test. Generally, the test access of a telephone switch can reliably pass signals up to around 20 KHz. However, a DSL system often uses signals over 1 MHz. And, even when the EWL is computed from attenuation at a particular frequency, that single frequency is likely to be in the 100 kHz to 300 kHz range. In addition to requiring a single ended measurement technique, there is a particular challenge making a single ended measurement through a switch, which limits the range of frequencies that can be used for the measurement to be below the frequency at which attenuation needs to be measured.
The Celerity(trademark) pre-qualification system has solved this problem. It uses a range of techniques using low frequency measurement signals to pre-qualify a line for high speed data services. The maker of that product, Teradyne, Inc., is the assignee of several patents and patent applications relating to line pre-qualification. PCT application WO 01/67729, entitled xe2x80x9cTechnique for Estimation of a Subscriber Line Insertion Lossxe2x80x9d describes a technique to estimate insertion loss at a single frequency. Insertion loss can be directly related to line attenuation. The maker of that product also owns the following patents and applications: PCT application WO 00/27134 entitled xe2x80x9cMethod and Apparatus for Qualifying Loops For Data Servicesxe2x80x9d; PCT application WO 00/64132 entitled xe2x80x9cPredicting Performance of Telephone Lines for Data Servicesxe2x80x9d; PCT application WO 01/01597 entitled xe2x80x9cQualifying Telephone Lines for Data Transmissionxe2x80x9d; and PCT application WO 01/24490 entitled xe2x80x9cSubscriber Line Qualification with Numeral Networks with Respect to Data Transmission Characteristics.xe2x80x9d The foregoing patents and applications are all hereby incorporated by reference.
With the foregoing background in mind, it is an object of the invention to provide an improved technique to pre-qualify a line for high speed data services using one-ended measurements.
To achieve the foregoing object, as well as other objectives and advantages, a system is provided that uses low frequency measurements on a telephone line to predict line attenuation at multiple high frequencies and then providing a more accurate characterization of the line based on an average loop loss.